The proposed longitudinal and repeated cross-sectional study, Project EAT-IN, aims to enhance our understanding of environmental, personal, and behavioral factors that influence weight status and related behaviors, including weight control behaviors, dietary intake, and physical activity, during adolescence and young adulthood. The primary research questions to be addressed in Project EAT-III are: 1) How do weight status and related behaviors change longitudinally over 10 years, as youth progress through adolescence and young adulthood and go through key developmental and transitional periods? 2) What secular changes in weight status and related behaviors have occurred over the past 10 years? 3) What are the most potent environmental, personal, and behavioral influences on weight status and related behaviors? 4) How do different layers of environmental, personal, and behavioral influences work together to affect weight control behaviors, dietary intake, physical activity, and, in turn, weight status? 5) How do weight status and related behaviors compare with the Healthy People 2010 Objectives, as we approach the year 2010? 6) What disparities in weight status and related behaviors and influencing factors are evident across gender, ethnicity/race and socio-economic status? Project EAT-III will build upon the previous Project EAT-I and EAT-II studies. In 1999, Project EAT-I collected survey and anthropometric data on a large, ethnically and socio-economically diverse sample of adolescents. In 2004, Project EAT-II followed up on the original participants. Projects EAT-I and EAT-II have been very informative and have resulted in 72 scientific papers. In order to examine 10-year longitudinal trends and learn more about the young adult population, the previously established Project EAT study population will be followed (N=2400 early and middle young adults). In order to examine secular trends between 1999-2010 and explore a wider range of neighborhood, school, and peer influences than were assessed in previous study waves, a new cohort of adolescents will be recruited (N=2400 early and middle adolescents). Project EAT-III will provide data on how weight status, weight control behaviors, dietary intake, and physical activity change from earlier to later periods of adolescence and young adulthood and the multiple layers of influences on these outcomes. Findings will increase our theoretical understanding of the long-term predictive factors influencing weight status and related behaviors and will guide the development of more effective interventions to promote healthy eating and physical activity and reduce the prevalence of obesity.